Replace Van Den Hul First Ultimates with silver IC's ???

Fiddler, I use the VDH 1st Ultimates in my system too. I know what you mean about their ability to retrieve detail. However, they lack a bit of speed and transparency compared to silver cables. In my opinion the best of both worlds (the harmonic detail of the VDH and the transparency of silver wire) can be obtained by using one of each in your system. I have 1/2 meter of KCAG from source to preamp, followed by 1 meter of Ultimate to the amp. I have compared many, many cables in combination and always return to this pair as the most satisfying. The Ultimate is an terrific bargain, which I would not sell at least until you've had a chance to compare for yourself. As for sibilance, I would look elsewhere for the cause--the Ultimates are among the most natural sounding cables--they are no more capable of producing sibilance than silver wire is likely to sound flat or dull. Ensure all your connections are secure and look for a good line conditioner if needed. You already have a great cable, don't succumb to needless equipment swapping. Hope this helps.

I use VDH First Ultimate between CDP and Preamp.
I hear some sibilance, but it is part of the digital recording process. This can easily be verified by listening to the same Cd on a portable CD player with good headphones (Sennheiser 600).

I have tried silver cables in place of the First. In general, you will hear more treble and increased sibilance with most silver cables. There may be exceptions, which I have not auditioned. The First ultimate sounds very smooth and has a magic way of reducing most of the digital "nasties".
I suggest that you look elsewhere for the source of sibilance. Maybe an aged CDP or stranded loudspeaker cables ?
Good luck